For diabetes treatment, management of the blood glucose level by a patient himself or herself is important for maintaining the blood glucose level in a normal range. Particularly, measuring the blood sugar level regularly is essential for a patient of insulin-dependent diabetes to maintain the blood sugar level in a normal range. However, it is troublesome to often go to a medical institution for measuring the blood glucose level. Thus, portable blood glucose level measuring apparatuses are in practical use, which makes it possible to measure the blood glucose level without going to a medical institution.
As an example of such portable blood glucose level measuring apparatuses, the gazette of JP-A-2000-231discloses a concentration measuring apparatus used by attaching a mounter to a main body of a blood glucose measuring apparatus. As shown in FIG. 11 of the present application, the mounter disclosed in the gazette includes a lancing member 70 provided with a lancing needle 70a, and a biosensor 71 retaining enzyme and integrally connected to the lancing member. The lancing needle 70a is accommodated in a space 73 closed by the biosensor 71 as exposed in the space. The biosensor 71 is formed with a through-hole 71a for allowing the movement of the lancing needle 70a. Therefore, in the mounter 7, the lancing needle 70a is exposed whether or not the biosensor 71 is integrally connected to the mounter.
From a hygiene point of view, the lancing needle 70a need be sterilized, and the sterilized state need be maintained until the use of the needle. For this purpose, the lancing needle 70a need be sterilized in a hermetically closed space and the closed state need be maintained until the use. As described above, in the mounter 7, the lancing needle 70a is exposed whether or not the biosensor 71 is integrally connected. Therefore, in the mounter 7, the sterilization of the lancing needle 70a need be performed in the state enclosed by an aluminum laminated film or the like. Therefore, in the mounter 7, the sterilization of the lancing needle 70a cannot be performed in the state separated from the biosensor 71 and hence from enzyme retained in the biosensor 71.
The gazettes of Japanese Patent No. 2616331 and JP-A-9-94231 disclose lancing needles having biosensing function. Japanese Patent No. 2616331 discloses such a biosensor 8 as shown in FIG. 12 of the present application. The biosensor 8 includes a hollow lancing needle (counterpart electrode) 80, and a measurement electrode (operative electrode) 82 which is accommodated in the lancing needle 80 and in which enzyme 81 is fixed. The gazette of JP-A-9-94231 discloses such a biosensing device 9 as shown in FIG. 13 of the present application. The biosensing device 9 includes a lancing needle 90 integrally connected to a capillary 91. In the capillary 91, an operative electrode 92 in which enzyme is fixed, a counterpart electrode 93, and a reference electrode 94 are disposed. Also in the structures disclosed in these gazettes, the sterilization of the lancing needles 80, 90 cannot be performed in the separated state from enzyme.
When the above-described biosensors (biosensing device) are used, the measurement of the blood glucose level is performed using the amperometric method, for example. Specifically, a reaction field is first formed by causing enzyme to coexist with an electron carrier (e.g. potassium ferricyanide). Then, blood is supplied to the reaction field for oxidizing glucose contained in the blood while reducing the electron carrier. Thereafter, voltage is applied to the reaction field to oxidize the electron carrier, and the emitted amount of oxygen is measured as oxidation current. The blood glucose level is computed based on the value of the oxidation current.
The sterilization of the lancing needle may be performed by the radiation of gamma rays, for example. Therefore, when enzyme exists in the sterilizing process of the lancing needle, the enzyme is deactivated or the activity of the enzyme is deteriorated due to the gamma rays radiation. In such a case, a longer time may be required for the measurement or the measured concentration may be lower than the actual concentration, so that proper measurement cannot be performed.
On the other hand, when an electron carrier exists in the sterilizing process of the lancing needle, the electron carrier may be reduced. In such a case, the reaction field includes the reduced electron carrier derived from gamma rays radiation in addition to that derived from enzyme reaction. As described above, in the amperometric method, the blood glucose level is computed based on the amount of electrons emitted from electron carriers. Therefore, when an electron carrier is reduced by gamma rays radiation, the oxidation current value actually measured in applying voltage becomes higher than the value which should be measured. As a result, a concentration value higher than the actual concentration is computed, so that proper measurement cannot be performed.